Zn- and Fe-doped YBCO systems were investigated as a function of the dopant concentration and the oxygen stoichiometry and with respect to the resistive characteristics, bulk simmetry and oxygen microdomain ordering. It was verified that Zn, while strongly abating Tc, is not inducing any appreciable modification in the bulk simmetry and the oxygen ordering with respect to the undoped YBCO. On the contrary, Fe substitution, which is less effective on Tc at comparable concentration level, heavily modifies the structural simmetry and hinders the formation of oxygen ordered superstructures on a scale range appreciable by the electron diffraction technique.